U.S. Severe Weather Forecast (Coup d'œil sur la convection)

Day Three

acus01 kwns 210535 
Storm Prediction Center ac 210533 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1233 am CDT Thursday Aug 21 2014 

Valid 211200z - 221200z 

..there is a slight risk of severe thunderstorms from the upper MS valley to the 
Ohio Valley... 

Scattered severe thunderstorms are expected to develop from portions 
of the upper Mississippi Valley into the Ohio Valley. Damaging 
winds and large hail are the primary threats with this activity. 

..upper MS valley/Ohio Valley... 

Early morning WV imagery depicts a transitory short-wave trough 
ejecting eastward across the central Dakotas. This feature is expected to 
top the ridge over the upper MS valley early in the period before 
turning southeastward into the upper Great Lakes region. Considerable amount 
of convection is noted ahead of this feature at midnight from southeastern 
ND into north central South Dakota. This activity should progress into southern Minnesota by 

Latest model guidance supports the progression of short wave into Minnesota 
by daybreak and surface warm front draped across Minnesota/southern WI is expected 
to move little through the period. For this reason early morning 
thunderstorms will likely not dissipate prior to diurnally-enhanced 
destabilization. Modest northwesterly flow...on the order of 30kt at 
500mb...appears adequate for upward growth of thunderstorms as they track 
east-southeastward along warm front. Forecast soundings suggest moderate 
buoyancy will develop across much of the warm sector from southeastern South Dakota 
into the Ohio Valley where SBCAPE should easily exceed 3000 j/kg. 
There is some concern that warm frontal convection could organize 
into more expansive thunderstorm clusters that could surge southeastward into 
the Ohio Valley due to contributions of forward propagation. At this 
time will only introduce 15 percent severe probs for wind and hail 
as it/S not real clear how this convection will ultimately evolve. 

Farther west across the Central Plains...Post short wave airmass is 
forecast to destabilize significantly from southeastern South Dakota into eastern Nebraska 
where SBCAPE could approach 4000 j/kg. WV imagery and model 
guidance do not suggest any meaningful disturbance into this region 
during the afternoon/evening hours. For this reason it appears weak 
low level convergence and diurnal heating will be the primary 
mechanisms for potential isolated thunderstorm development. Weak 
warm advection atop the boundary layer may also contribute to 
elevated convection after dark...mainly across Iowa. Large hail/gusty 
winds are the main threats with this activity. 

..lower Colorado River valley... 

00z NAM guidance suggests weak upper low over Southern California will drift east 
toward the lower Colorado River valley. A belt of modest southwesterly flow at 
500mb is forecast across northwestern Mexico into extreme southern Arizona. While 
thermodynamic profiles are not particularly unstable over the higher 
terrain of Arizona a small pocket of buoyancy may evolve beneath the 
upper low. If sufficient instability develops near the Mexican 
border then a few strong thunderstorms may ultimately evolve during 
the late afternoon/early evening hours. 

.Darrow/Mosier.. 08/21/2014 

Mesoscale Discussion

acus11 kwns 210530 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 210529 

Mesoscale discussion 1591 
National Weather Service Storm Prediction Center Norman OK 
1229 am CDT Thursday Aug 21 2014 

Areas affected...far eastern South Dakota into central/southern Minnesota 

Concerning...severe potential...watch possible 

Valid 210529z - 210730z 

Probability of watch issuance...60 percent 

Summary...the potential for at least isolated bouts of large hail 
are expected to increase overnight from far eastern South Dakota into 
central/southern Minnesota. Monitoring convective trends for a possible 

Discussion...elevated strong/severe thunderstorms continue to 
increase near/just east of the South Dakota/Minnesota border vicinity as of 
0515z...with these storms preceding a small/loosely organized 
convective complex across northeast South Dakota. Near the crest of a 
shortwave ridge...this increase in elevated convective development 
is being aided by a relatively strong low-level jet and unstable 
source region air mass. Latest WSR-88D vwp data from Sioux Falls 
South Dakota/Omaha Nebraska are sampling a 40 knots southwesterly low-level jet...with 
an earlier 00z observed sounding from Omaha sampling a very unstable 
source region with nearly 7.5 c/km middle-level lapse rates and 5000+ 
j/kg most-unstable cape. As warm advection/isentropic ascent 
supports an increasing number of storms...the strongest activity 
will be capable of large hail across central/southern portions of 
Minnesota. While large hail is the primary risk given a relatively stable 
boundary layer...locally strong wind gusts cannot be entirely ruled 
out...especially if storm mergers/upscale trends occur over time. 

.Guyer/carbin.. 08/21/2014 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 46579487 45619346 43929312 43629531 44939730 45959716